Lenses that are typically used in personal protection equipment—for example, full face piece respirators, welding helmets, and face shields—are commonly made from polycarbonate resins. These resins provide excellent visual clarity and demonstrate extraordinarily good impact resistance. Polycarbonate lenses, however, exhibit a particular drawback in that they can become easily scratched and may fracture when exposed to some common chemical solvents. Consequently, polycarbonate lenses are often coated with a protective silsesquioxane-based hard-coat composition, such as a methyltrimethoxysilane hydrolyzed in a mixture of water and alcoholic solvent and in the presence of a colloidal silica, an example of which is commercially available under the trade designation GE SHC 1200 from GE Silicones, (Waterford, N.Y.), to provide protection against both scratches and chemical crazing.
Another problem for workers who wear personal protection equipment, such as full face piece respirators, is that paint and other materials are splashed onto the lens, creating a staining pattern that can obstruct the wearer's vision. Common practice is to wipe the stain using a solvent-soaked rag. If the lens includes a protective hard-coat, the solvent may cause damage to the hard-coat, resulting in a shortened service life for the lens, or the solvent may wear away a portion or all of the hard-coat.
Hard-coat layers have been applied to a variety of substrates to increase the substrate's resistance to abrasion and degradation due to chemical exposure. However, known hard-coats have not addressed problems pertaining to stain resistance and cleanability.